I need to make a plugin that will take effect the user chooses

I am developing a jquery plugin and the problem I have is that when trying to pass settings, works with two of the three options I try to set. When I want to pass the "fadeIn" effect does not display me anything and stops working. This is my code:

If you don't use brackets, Javascript is going to look for a configuracion property on the next('div') object, with an efecto method attached to it (which isn't the case). Using brackets works out to .next('div')['fadeIn'](configuracion.velocidadEfecto) because configuracion.efecto is evaluated to its string value.